Amorphous silicon (a-Si) is a form of silicon that has no crystalline structure. A-Si is critical to producing thin film transistors (TFTs) because, due to its lack of crystal structure, it can be vapor-deposited onto large substrates. However, the elimination of the crystal structure significantly compromises the performance of a-Si as a TFT semiconductor material. Polycrystalline silicon or polysilicon (p-Si) is a material consisting of small silicon crystals. TFTs made with p-Si have a higher mobility than that of TFTs made with a-Si due to the crystalline structure of the p-Si. The higher mobility of p-Si TFTs provides a higher aperture ratio when used as switch transistors for display pixels. In addition, the higher mobility of p-Si TFTs offers the possibility of integrating peripheral driving circuits with the display pixel matrix, thus enabling a system on panel (SOP). Accordingly, p-Si TFTs are excellent candidates for the next generation flat-panel displays, particularly those based on organic light-emitting diodes. As users demand higher quality in flat-panel displays with larger viewing area and finer resolution, the p-Si TFT technology becomes increasingly important relative to the a-Si TFT technology currently employed in color active matrix liquid crystal displays.
The quality of the p-Si layer of a device employing p-Si TFTs is one of the key factors which determines the performance of the device. Much effort has been made to improve the quality of p-Si used in TFTs while balancing increased manufacturing costs relative to that of a-Si. P—Si can be directly deposited onto a TFT substrate by means of chemical vapor deposition (CVD). However, CVD deposition results in a high defect density, lowering the mobility of the p-Si. An alternative and preferred technique is to crystallize a-Si to form p-Si.
The three broad categories of a-Si crystallization methods include solid-phase crystallization (SPC), metal-induced crystallization (MIC) and excimer laser crystallization (ELC). The conventional SPC process is the simplest and most direct method to obtain p-Si. With SPC, after CVD deposition of a-Si onto a substrate, the substrate is heat-treated at a specific temperature for a specific duration to crystallize the a-Si. The MIC process involves usage of a metal catalyst to facilitate crystallization of a-Si during heat-treatment. MIC generally requires a shorter processing time and/or a lower processing temperature relative to that of SPC. ELC employs a laser as a local heat source to induce the crystallization a-Si. Although ELC has been determined to produce a higher quality p-Si relative to that of conventional SPC or MIC, ELC requires high cost equipment and provides a slower process throughput. In addition the p-Si produced with ELC has inferior uniformity relative to the p-Si produced using SPC or MIC. Accordingly, there is a strong business incentive to develop new MIC and/or SPC techniques that produce high quality p-Si while maintaining the low manufacturing cost afforded by MIC and SPC.